1. Field of the Invention
The present invention relates to a magnetooptical computer disk having one or more optically transparent dimensionally stable substrates and one or more double layer systems consisting of two exchange-coupled magnetic layers which consist of alloys of rare earth metals with transition metals and have vertical magnetic anisotropy. The present invention relates in particular to magnetooptical computer disks having a Pt-containing reproducing layer.
2. Description of the Related Art
Known magnetooptical recording layers for recording and reading information are, for example, monocrystalline garnet layers (e.g. yttrium iron garnet), polycrystalline layers of MnBi or amorphous layers of alloys of lanthanides (RE) and transition metals (TM), abbreviated below to (RE-TM).
Recently, the amorphous (RE-TM) layers have been preferred since these recording layers can be produced over large areas by sputtering methods or vapor deposition methods, and the recorded signals can be read with a high signal-to-noise ratio. Many amorphous (RE-TM). alloys, for example Tb-Fe, Tb-Fe-Co, Gd-Tb-Fe-Co, Dy-Fe-Co, Nd-Tb-Fe-Co or Nd-Dy-Fe-Co, additionally have the advantage that the ferrimagnetic coupling of the RE and TM atoms results in a high coercive force in a direction at right angles to the plane of the layer.
These known magnetooptical computer disks are used for recording or writing data with the aid of laser beams (for example pulse-modulated), which are focused on the magnetooptical recording layers and strike them at right angles.
During recording or writing of data, an external auxiliary magnetic field is applied to the magnetooptical computer disks, the field lines of which field are oriented at right angles to the surface of the magnetooptical recording layers. The direction of the external magnetic field is opposite to the direction of magnetization of the magnetooptical recording layer. In addition, the magnetooptical recording layers may have a correspondingly oriented immanent (intrinsic) magnetic field. In a known alternative recording method, the external magnetic field is time-modulated.
It is known that the magnetooptical recording layers which consist of amorphous ferrimagnetic (RE-TM) alloys, are magnetized at right angles to their surface and may have a plurality of layers are heated at the point of contact during recording of data by the write laser beam. As a result of the heating, the coercive force H.sub.c of the alloys decreases. If the coercive force H.sub.c falls below the sum of the field strengths of the applied (external) auxiliary magnetic field and of the intrinsic field at a critical temperature dependent on the particular alloy used, a region which has a direction of magnetization opposite to the original direction is formed at the point of contact. Such a region is also referred to as a magnetic domain.
The diameter and the shape of the domains formed depend both on the size of the laser spot, the laser power, the laser pulse time and the strength of the external magnetic field and on the magnetization M.sub.s and the coercive force H.sub.c of the recording layer. Round smooth-edged domains are desirable since they give a high signal and a high signal-to-noise ratio during reading.
In the write process, smooth-edged domains are obtained in particular when the magnetization and the coercive force of the magnetooptical storage layer have a suitable temperature dependence and the Curie temperature T.sub.c of the storage layer is at least approximately reached during heating by the laser beam.
At temperatures substantially above T.sub.c, large, overlapping domains having poor signal-to-noise ratios are obtained. On the other hand, at temperatures substantially below T.sub.c, the nucleation of domains having the opposite magnetization is possible only with very high external magnetic fields which are therefore unsuitable for use.
It is known that, during recording of the data, the write laser beam is moved relative to the magnetooptical computer disk or its magnetooptical recording layer and above the surface of said disk or layer. In general, the laser beam is focused on the recording layer by a displaceable optical apparatus, and the relevant magnetooptical computer disk is rotated at constant angular velocity (CAV).
It is known that the data recorded in the magnetooptical computer disks can, if required, be deleted by controlled local heating of their magnetooptical recording layer, for example by means of an unmodulated continuous laser beam with the simultaneous action of an external or an intrinsic magnetic field whose field lines are oriented at right angles to the surface of the recording layer, after which further data can be recorded, i.e. the write process is reversible.
The data are usually read using the linearly polarized light of a continuous-wave laser whose power is not sufficient to heat the material above the critical temperature. This laser beam is reflected either by the recording layer itself or by a reflector layer arranged behind it, the result being an interaction between the magnetic moments in the recording layer and the electromagnetic field of the laser light. Because of this interaction, the plane of polarization of the reflected laser light is rotated through a small angle relative to the original plane. If this rotation of the plane of polarization occurs when the light is reflected by the recording layer itself, the term Kerr effect is used and the angle of rotation is accordingly referred to as the Kerr angle; if, on the other hand, the plane is rotated during passage of the light twice or a greater number of times through the recording layer, the terms Faraday effect and Faraday angle are used. The direction of rotation of the plane of polarization depends on the magnetization direction at the relevant point of the storage layer. This rotation of the plane of polarization of the laser light reflected by the magnetooptical computer disk can be measured with the aid of suitable optical and electronic apparatuses and converted into signals, as described in, for example, U.S. Pat. No. 4 466 035. The magnetooptical read signal is proportional to the product of the Kerr angle and the reflectivity of the magnetooptical layer system. A high Kerr angle accordingly results in a high read signal and a correspondingly improved signal-to-noise ratio.
For the laser wavelengths currently used (from 780 nm to 830 nm), the Kerr angle of the stated (RE-TM) alloys is, as a rule, from 0.2.degree. to 0.3.degree.. The Kerr angle of alloys based on the heavy RE elements Gd, Tb and Dy generally decreases with decreasing wavelength. Since it is expected that short-wavelength lasers will be used for future magnetooptical computer disks, the stated (RE-TM) alloys have the disadvantage of a reduced magnetooptical read signal.
As described in the stated U.S. Pat. No. 4,466,035, the Kerr angle can be increased by using suitable dielectric layers on the front and back of the magnetooptical recording layer and by employing a metallic reflector on the back of the magnetooptical recording layer. When the Kerr angle is increased with the aid of dielectric layers, a decrease in the reflectivity must, as a rule, be accepted. Since a minimum reflectivity is required for operating a magnetooptical storage medium in a prior art drive, the increase in the magnetooptical read signal in the manner described is subject to limits. In particular, the stated disadvantageous reduction of the Kerr angle with decreasing wavelengths of the recording laser cannot be avoided by optical matching with dielectric layers.
It is known that the Kerr angle of an amorphous (RE-TM) layer can be increased by alloying with Pt. At the same time, however, considerable reduction in the coercive force in the direction at right angles to the film surface is observed. In the known magnetooptical recording layers, the advantage of the increased Kerr angle can therefore often be utilized to a limited extent since the Pt concentration must be kept low because of the coercive force
Another great disadvantage of the stated (RE-TM) alloys is their poor corrosion resistance. Direct contact of the layers with air or water vapor results in progressive oxidation of the magnetooptical layer over a large area, said oxidation initially causing a reduction in the Kerr angle and in the reflectivity and hence a decrease in the signal-to-noise ratio and finally leading to completely oxidized layers which are useless for magnetooptical purposes.
A possible method for improving the corrosion resistance of magnetooptical computer disks based on (RE-TM) alloys is the application of a transparent protective layer on the front and back for avoiding direct contact of the recording layer with air and for inhibiting the entry of oxygen or water molecules by diffusion. This is possible only by means of very dense, crack-free and pore-free layers, for example of Si.sub.3 N.sub.4 or AlN. However, the additional deposition of the transparent protective layers before and after application of the recording layer makes the production process for magnetooptical computer disks substantially longer and more expensive. Furthermore, defects in the protective layer, for example pinholes or cracks, can lead to corrosion of the lower-lying magnetooptical recording layer. An expensive quality control of the deposited protective layers is therefore necessary in order to ensure their protective effect.
Owing to the high sensitivity of (RE-TM) alloys to oxidation, reactions of said alloys with reactive gases in the residual gas may also occur in the coating chamber. Particularly during the period after application of an (RE-TM)-containing layer and before application of the subsequent layer (for example, due to the changing of the sputtering target or transport of the coated disk), a superficial oxide layer may form as a result of reaction with the O.sub.2 or H.sub.2 O molecules of the residual gas.
In the known exchange-coupled double layer systems described further below, there is a weakening of the exchange interaction at room temperature between the reproducing layer and the storage layer owing to nonmagnetic intermediate layers which form after production of the first magnetic layer and before application of the second magnetic layer by a reaction of the (RE-TM) alloy with reactive gases in the residual gas of the coating unit. This exchange interaction is even decisively reduced by monolayers of a nonmagnetic intermediate layer, for example of a metal oxide. Although the superficial oxide layer can be substantially removed by etching processes, for example a plasma etching process in an Ar atmosphere, the production process for the magnetooptical computer disk is however made longer and more expensive as a result.
An alternative method for improving the corrosion behavior of amorphous (RE-TM) alloys is the alloy of corrosion inhibitors, i.e. elements which delay the corrosion of the recording layer. A number of (RE-TM) alloys with homogeneously alloyed corrosion inhibitors are known.
For example, EP-A 229 292 describes a magnetooptical recording medium which consists of a (RE-TM) alloy containing an additional element, for example Ti, Cr, Al, Pt, Zr, V, Ta, Mo, W, Cu, Ru, Rh, Pd, Nb, Ir or Hf. The addition of said element delays the decrease in the coercive force and in the Kerr angle during storage of the magnetooptical layer in direct contact with humid air.
U.S. Pat. No. 4,693,943 describes a magneto-optical recording medium having an amorphous (RE-TM) alloy with the composition [(dTb).sub.1-y (FeCo).sub.y ].sub.1-p Cr.sub.p, where 0.5.ltoreq.y.ltoreq.0.9 and 0.001.ltoreq.p.ltoreq.0.3. The addition of Cr substantially improves the corrosion stability of the magnetooptical recording medium. We have found that the corrosion stability increases monotonically with increasing Cr content.
It is also known that the corrosion stability of (RE-TM) layers can be further improved by simultaneously alloying a plurality of elements with said layers. EP-A 302 393 describes a magnetooptical recording medium containing a (RE-TM) alloy with which from 1 to 10 atom % of one or more elements from the group consisting of Nb, Ti, Ta, Cr and Al and from 2 to 10 atom % of one or more elements from the group consisting of Pt, Au, Pd and Rh are also alloyed.
A substantial disadvantage of the use of corrosion inhibitors is that the magnetic and magnetooptical properties of the recording medium are as a rule adversely affected by alloying with a corrosion inhibitor. In many cases, the Kerr angle is reduced and the temperature dependence of the magnetization and of the coercive force are unfavorably changed, having adverse effects on the write and read behavior of the recording medium.
Exchange-coupled double layer systems which contain a first magnetic layer having a low coercive force and a second magnetic layer having a high coercive force are described in, for example, EP-A 51 296, U.S. Pat. No. 4,628,485, U.S. Pat. No. 4,753,853, EP-A 305 185, EP-A 330 394 and EP-A 333 467.
EP-A 51 296 describes a thermomagnetic recording medium having a first and a second magnetically anisotropic layer, the second magnetic layer having a higher coercive force and a lower Curie temperature than the first magnetic layer. The first magnetic layer essentially contains a Gd alloy
U.S. Pat. No. 4,628,485 describes a magnetooptical recording medium having a first thin magnetic layer of low Curie temperature and high coercive force (recording layer), an adjacent second magnetic layer (reproducing layer) having a high Kerr angle and further dielectric and metallic layers for optically increasing the Kerr angle.
U.S. Pat. No. 4,645,722 describes a magnetooptical recording medium having a first magnetic layer which possesses a high coercive force and a second, multi-stratum magnetic layer system which has a higher Kerr angle and/or a higher reflectivity than the first magnetic layer.
U.S. Pat. No. 4,753,853 describes an exchange-coupled magnetooptical double layer system in which a first layer has a lower Curie temperature and a high coercive force and consists of a TM-rich (Gd-Fe-Co) alloy. The second layer has a high Curie temperature and a low coercive force and consists of a TM-rich (Tb-Fe) alloy.
EP-A 330 394 discloses a magneto-optical recording medium of the double layer type whose magnetic layer having a low coercive force and high Curie temperature contains Gd and one or more of the two elements Tb and Dy. The coercive force of the two magnetic layers and the ratio of the domain wall energy between these two layers to the product of saturation magnetization and thickness of the layer having a low coercive force must satisfy specific conditions.
The magnetic layer having a low coercive force may be doped with one or more elements from the group consisting of Ni, Cr, Ti, Al, Si, Pt, In and Cu. However, no data at all is given with regard to the concentration of these elements Moreover, EP-A 330 394 does not disclose the purpose for which the layer having a low coercive force is to be doped with these elements.
EP-A 364 212 describes a magnetooptical recording medium having a first magnetic layer (reproducing layer) of an amorphous R.sub.1 -Fe-Co-Cr alloy, where R.sub.1 is one or more elements from the group consisting of Tb and Dy, and a second magnetic layer (recording layer) of an amorphous R.sub.2 -Fe-Co-Cr alloy, where R.sub.2 is one or more elements from the group consisting of Tb, Dy and Gd. The proportion of Co in the first magnetic layer is smaller than the proportion of Co in the second magnetic layer.
EP-A 364 196 describes a magnetooptical recording medium which is very similar to that in EP-A 364 212, except that the proportion of Cr in the first magnetic layer is greater than the proportion of Cr in the second magnetic layer.